In the manufacture of semiconductor devices, alloys of tungsten and titanium are used as diffusion barriers between silicon substrates and aluminum alloy metallizations. The tungsten-titanium ("W-Ti") thin film barriers are formed by sputtering W-Ti targets. In general, these targets are fabricated by mixing starting powders of tungsten and titanium and applying heat and pressure to the mixed powders using a forming method such as inert gas hot pressing, vacuum hot pressing, hot isostatic pressing, and cold pressing/sintering.
One problem associated with known W-Ti targets is particulate emission by the target during sputtering. Particulate generated during sputtering contaminates the thin film and thus negatively affects the reliability and productivity of the thin film generated, causing manufacturing yield losses. Accordingly, there has been considerable interest in evaluating causes of particulate generation during the sputtering of W-Ti targets and minimizing target particulate emission. Because most conventional W-Ti target structures consist of multiple phases, including tungsten, titanium, and .beta.(Ti,W), most studies have evaluated particulate generation by multiple phase W-Ti targets.
Studies of particulate emission by multiple phase targets have concluded that particulate generation is determined by the method of forming the target, the density of the target, or the percentage of .beta.(Ti,W) phase in the target structure. For example, early studies indicated that vacuum hot processing was the preferred technique for forming multiple phase targets that produced low particulate during sputtering. Later investigation of multiple phase targets indicated that low particulate generation was a function of a higher target density. U.S. Pat. No. 5,234,487 teaches that decreased particulate emission from multiple phase W-Ti targets can be achieved by having substantially zero .beta.(Ti,W) phase in the target structure. U.S. Pat. No. 5,234,487 further teaches that .beta.(Ti,W) phase is responsible for high particulate emission due to the fact that the .beta.(Ti,W) phase is hard and brittle.
The .beta.(Ti,W) phase consists of finely dispersed tungsten within titanium. This .beta.(Ti,W) phase forms by the interdiffusion of W into Ti during high temperature processing of W-Ti target materials and is the result of a solid-state precipitation, a eutectoid reaction. The phase diagram for tungsten-titanium indicates that at temperatures above 740.degree. C., a 10 weight percent titanium alloy will consist of single .beta.(Ti,W) phase solid solution. To achieve equilibrium for the W-Ti system, the 10 weight present W-Ti alloy must be held at a temperature above 740.degree. C. for a sufficient time to homogenize by diffusion the titanium and tungsten powders. Lower temperatures require longer holding times to achieve equilibrium. Conversely, higher temperatures require less time to achieve equilibrium.
In the manufacture of tungsten-titanium sputtering targets, equilibrium typically is not achieved because tungsten and titanium are not uniform throughout the microstructure of the target. Diffusion of tungsten into regions of titanium tends to produce areas represented by the titanium-rich region of the phase diagram. As these titanium-rich areas cool, the solubility of tungsten in titanium drops, precipitating tungsten. This fine distribution of tungsten in the surrounding titanium is the .beta.(Ti,W) phase. As a consequence of the failure to achieve equilibrium, a resulting typical multiple phase target microstructure consists of three phases--Ti, W, and .beta.(Ti,W). The formation of a single .beta.(Ti,W) phase target has been difficult to accomplish. Consequently, the properties of single phase .beta.(Ti,W) targets and the parameters affecting particle generation by such targets are not well understood.
Despite the extensive research to better understand the relationship among multiple phase W-Ti target manufacturing processes, microstructure, and particulate emission, there is still a need for a method of producing single .beta.(Ti,W) phase sputtering targets. Furthermore, there is a need for W-Ti sputter targets that emit less particulate during sputtering than conventional multiple phase W-Ti targets.